1. Field of Invention
The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method for integrating a high-voltage device and a low-voltage device.
2. Description of Related Art
Typically, in a low-voltage logic circuit, it is necessary to use a high-voltage device to transfer the proper voltages into different electronic devices. The high voltage device is located at the interface between the low-voltage logic circuit and the electronic devices. Hence, in order to decrease the cost and to obtain the demand voltages for driving different electronic devices, it is important to develop a method for integrating a low-voltage device and a high-voltage device.
Conventionally, the method of forming a high-voltage device comprises the steps of forming a polysilicon gate on a substrate, and then using the polysilicon gate as a mask and forming a source/drain region with a double-diffused drain (DDD) structure in a self-aligned process. Commonly, in order to suppress the hot electron effect and to increase the breakdown voltage in the source/drain region, a lightly doped region is formed in the substrate under the source/drain region and the isolation region, and then a high-temperature drive in process is performed to form the DDD structure. Therefore, the high-voltage device can be normally operated under a high voltage situation. However, in the procedure for integrating the high-voltage device and the low-voltage device, the structures of the high-voltage device and the low-voltage devices and the heat budgets for the high-voltage device and the low-voltage devices are different. When the grade region, that is, the lightly doped region, is formed in the substrate and a drive in process is performed to form a DDD structure, the electrical property of low-voltage device fluctuates. Therefore, it leads to an electrical instability problem of the low-voltage device.